1. Technical Field
The present invention relates generally to the bonding or joining of metals. More particularly, the invention relates to joining metal parts by shear forge bonding, under controlled heat, atmosphere and pressure to promote high integrity metallic bonding of similar or dissimilar metals, but especially superalloys for dual alloy disks.
2. Background of the Invention
Ideally, if two perfectly clean and perfectly smooth metal surfaces are placed in contact with one another, a perfect metallic bond will result which will be indistinquishable from the native metal surrounding it. However, metal and metal alloy surfaces in normal atmospheric environments are always covered with bonding disruptive contamination, such as films of oxides, nitrides, and/or adsorbed gases, which prevent sufficient intimacy of contact to achieve metallic bonding.
Processes have been developed to eliminate surface films and establish the necessary intimacy of contact between metal parts. An example is fusion welding, which effectively disrupts surface films by dissolution and/or melting and establishes the necessary intimacy of contact by wetting in the molten phase. The problem with fusion welding is that the fused weld joint is very distinct from the wrought microstructure surrounding it, potentially adversely affecting the mechanical properties of the metal(s) at the joint or requiring post-joining treatments to ameliorate those adverse effects. Brazing and soldering are also examples of joining processes which disrupt surface films by dissolution or melting of surface contaminants; they establish the necessary joint by bridging components with filler material which forms intimate contact by wetting the molten phase on respective surfaces of the component parts. These lower temperature processes present less severe adverse consequences, but are also of more limited engineering application.
Another process for joining metals is diffusion bonding, where cleaned metal surfaces are bonded under high temperature and pressure in a vacuum or other benign environment. This type of bonding has been successful for some metal systems, such as titanium, iron or copper, which dissolve all or most of their usual surface contaminants. However, aluminum or superalloys which form tenacious surface oxides that do not dissolve or diffuse into the parent alloy require special processing, such as coating the surface to be bonded with boron or another joining adjuvant. Such a coating will clean surface oxides and dissolve into a film of molten metal during high temperature diffusion bonding, but can cause property debits in the joint.
Inertia or friction welding is a solid state welding process which uses heat of friction developed between rotating and stationary workpiece components to produce a metallic bond. Contaminants on the mating surfaces are dissolved, attrited and/or displaced with deformed surface metal beyond the final joint geometry. The equipment required is generally sophisticated, expensive, and amenable primarily to face-to-face joints.
It is desirable in metal bonding processes to clean the surfaces to be bonded of all contaminants and to maintain the surface integrity until the surfaces can be brought into intimate contact to achieve metallic bonding. Another desirable feature for many bonding processes is to bond the metal surfaces at forging or solution temperatures rather than ultra-high temperatures (i.e., temperatures above alloy solvus temperature). The above and other features are addressed by various aspects of the invention described below.